A microelectronic element, such as a semiconductor chip, is typically connected to an external circuit element through contacts accessible at a surface of the microelectronic element. For example, in the tape automated bonding process (hereinafter referred to as the "TAB" process), a flexible dielectric sheet, such as a thin foil of polyimide, includes conductive terminals accessible at a surface thereof and flexible metallic leads connected to the terminals. The flexible dielectric sheet also preferably includes one or more bond windows extending therethrough. Each flexible lead preferably has a first end integrally connected to one of the conductive terminal and a second end remote therefrom which projects beyond one of the bond windows. The flexible dielectric sheet is typically juxtaposed with a semiconductor chip so that the bond windows are aligned with contacts on a front end face of the chip and so that the second ends of the leads overlie the contacts. The flexible leads may then be bonded to the chip contacts using bonding techniques, such as ultrasonic or thermocompression bonding. After the bonding step, the chip package may be electrically interconnected with an external circuit element, such as a printed circuit board, by connecting the conductive terminals with contact pads on the external circuit element.
The electrical interconnections between the conductive terminals of the chip package and the external circuit element are typically made by using fusible conductive elements, such as solder balls. The solder balls are positioned between the conductive terminals on the chip package and the contact pads on the external circuit element and then reflowed by raising the temperature of the solder balls above a predetermined temperature, generally referred to as the melting point of the solder balls. The melting point is defined as the temperature at which the solder balls transform from a first solid or frozen condition to a second molten or at least partially liquid condition. Once the solder balls have transformed to the second at least partially liquid condition, the solder balls remain in that condition as long as the temperature is maintained at or above the melting point. After the conductive terminals of the chip package and the contact pads of the external circuit element have been electrically interconnected by the reflowed solder balls, the temperature of the solder balls may be reduced to a level below the melting point, whereupon the solder balls transform from the second at least partially liquid condition to the first solid condition. The refrozen solder balls both mechanically and electrically interconnect the chip contacts with the contact pads on the external circuit element.
Existing methods for placing solder balls on conductive terminals have encountered a number of problems. First, production rates have remained low because placing solder balls on microelectronic assemblies is a slow and time consuming process. In addition, a material known as flux often is used to facilitate the solder bonding process. The flux aids in removal of metal oxides and helps the molten solder to wet to the terminals. The flux typically has a pasty consistency and helps to hold solder balls on the terminals. The flux often comes in contact with a stencil used to align and place the solder balls atop the terminals. This may result in the flux becoming clogged in the stencil openings. Because the flux has adhesive-like properties this may result in some of the solder balls sticking to the stencil or the openings in the stencil, rather than passing completely through the stencil openings.
Another problem occurs when the solder balls placed in the stencil openings and resting on the terminals and flux protrude from the top surface of the stencil at the openings therein. The existence of high profile solder balls protruding at the top surface of the stencil may prevent other solder balls from moving freely across the top of the stencil in order to fill other openings therein. Moreover, solder balls which have been previously deposited in one of the stencil openings may become dislodged from the opening by other solder balls moving across the top of the stencil. The occurrence of any of these problems may result in the production of defective microelectronic packages, i.e. packages having one or more solder balls which are not properly secured over each conductive terminal.
Thus, there is a need for improved methods for placing conductive elements efficiently and reliably atop conductive terminals. There is also a need for an improved placement fixture for placing conductive elements atop conductive terminals so as to create durable and reliable electrical interconnections between microelectronic elements.